Basic flow, fluid mechanics, settle a heated discussion topic

[stephenamoralee]

We are currently trialling a silicone tubing with very small slits in to make a sprinkler system in around 2 metres of 4mm o.d 2mm i.d tubing there are 38 slices slicing through the first wall at a set 3mm long slit, note.. this is one continual piece of tubing arranged in a snake like shape, no branches. Being silicone we realise its a great valve. However the heated discussion is coming from a non fluid engineer that tells me the pressure will always be equal throughout, and the fluid should always be released equally through each slit, upon a pressure being reached by the fluid entering the tubing. i.e when the tubing is full and extra pressure is applied, the entrance pressure is not always equal as in our setup its an additive that is applied in a special way through a hand held syringe input. My own main findings is that fluid is released near to the input pressure and does not always have enough pressure to open the slits nearer the end of the silicone tubing.

Any explanation would be greatly appreciated

Thankyou

Stephen

 

[racookpe1978]

Your colleague is confusing the "ideal" static fluid pressure (all pressure is equal all around the inside of an enclosure, IF the fluid is not changing elevation) with the dynamic pressure actually occurring in this very small diameter open end "pipe".

consider the energy equilibrium of each 10 cm of the tubing: Is not energy "lost" due to friction with the wall, turbulence in each slit, and the kinetic energy of the lost mass at each slit? Thus pressure goes down as distance increases.

Also, ask him if the fluid would keep flowing if the end were raised higher than the inlet.

 

[IFRs]

Look at a HVAC fluid flow example. This is quite similar. Energy is lost at every slit.

 

[LittleInch]

The issue is going to be the amount of fluid coming out of the slits and what is the effective hole size versus your main header pipe.

This tube is so small that it is unlikely to have a constant pressure along the length. You need something like total hole size being about 10-15% of the header size before you get equal or near equal pressure along the entire length. Instead of your 2MM ID imagine if this was 200mm ID but the same size slit?

To a certain extent your colleague is correct, but only when there is no flow. As soon as flow occurs then frictional losses starts to come into play. My guess is that your slits open at slightly different pressures which complicates things even further, if indeed this is what is happening.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[stephenamoralee]

Sorry I have 4 children in my sole care as well as doing a degree as well as operating as an Industrial Designer so don't always get time to get back straight away, I do appreciate the replies. Not being an expert in any way in fluid flow, we can calculate in Solidworks to some degree but the silicone valves are the only issue here, this is why I came to this forum, to find people who really know their speciality.
Really do appreciate the feedback

 

[Artisi]

As you are trailing it what is the outcome so far- can you measure flow at the first slit on the input end compared to the farthest slit.
It is assumed the far end of the tube is blanked off.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[hacksaw]

if you are using the silicone rubber tubing, that I've used, you understand its flexibility, and you opening will be normally closed. You have to ask your associate to measure the slit width...

by the way for a sprinkler system you've describe, you really need to use another material and carry out your test with a pre-filled reservoir, test you test procedure before you can even trust you data... it wont be a prinkle system, but you can at least characterize the flow characteristics.

Sherlock is on the right track until he too was misled...